Bone is a dynamic tissue that remodels to grow, adapt to stress and maintain integrity. Two cell types control this process, the osteoblast (OB) and the osteoclast (OC), which synthesize and degrade bone, respectively. Pathologic inequality in bone remodeling favoring resorption over formation leads to diseases such as osteoporosis, rheumatoid arthritis and metastatic bone cancer. Currently, over 30 million Americans have low bone mass and nearly 1% of our population suffers from rheumatoid arthritis. The yearly health care cost burden of these diseases is immense. Currently approved therapeutics targeting OCs are inadequate, necessitating the discovery of new targets. To resorb bone, osteoclasts secrete hydrochloric acid. To prevent a reciprocal build up of cytoplasmic base, electroneutral exchange of bicarbonate for chloride occurs through an anion exchanger. The identity of this exchanger eluded identification until our recent report showing Solute carrier family 4, anion exchanger, member 2 (Slc4a2, Ae2) is absolutely required for osteoclast activity during development. In its absence, mice develop profound osteopetrosis. A recent publication has identified a nearly identical phenotype in cattle that lack SLC4A2. To date, only a handful of mutations have been identified that so dramatically curtail the ability of osteoclasts to resorb calcified tissue. Our preliminary data suggest that SCL4A2 plays an unexpected complex role in OC physiology. We have found that SLC4A2 deficiency not only prevents OCs from properly secreting acid and performing anion exchange, but also profoundly affects the organization of their cytoskeleton. Many important questions remain regarding the biology of SLC4A2 in the OC. We do not know whether SLC4A2 is important in bone remodeling beyond the developmental period or in the pathogenesis of inflammatory skeletal disease. The relative contribution of the cytoplasmic and transmembrane domains of SLC4A2 to the regulation of osteoclast biology is unknown. Moreover, whether human osteoclasts utilize SLC4A2 is undefined. Four specific aims are proposed to answer these questions: 1) Establish the OC-intrinsic role of SLC4A2 and resolve the contribution of osteopetrosis to the lethal phenotype of Slc4a2-/- mice; 2) Establish the requirement of SLC4A2 in a mouse model of inflammatory arthritis; 3) Perform a structure-function analysis of SLC4A2 in OCs and 4) Confirm a role for SLC4A2 in human OCs. I will take advantage of the unique environment in Boston to facilitate these studies. Collaborations have been established with local experts in anion exchange physiology, OC cell biology, mouse models of RA and RNA interference. By analyzing the function of a gene that so profoundly affects OC biology, this grant will advance basic understanding of skeletal remodeling, make important contributions to general cell biology and signal transduction and, most importantly, define a new target to suppress pathologic bone loss.